Originally, road milling equipment was used to smooth out bumps on the surface of a roadway or to grind down the joinder of two adjacent concrete slabs that may have buckled. Now, these road milling machines are also used for completely degrading concrete and macadam roads down to their gravel base. Additionally, the road milling equipment can be used for trenching and mining operations. The combinations of bit assemblies have been utilized for a wide variety of operations, such as to remove material from the terra firma, such as degrading the surface of the earth, minerals, cement, concrete, macadam or asphalt pavement.
Road milling, mining, and trenching equipment are operated using a rotatable, cylindrical drum that includes a plurality of bit holder blocks mounted onto the drum in a herringbone, V-shape, or spiral configuration. Bits are traditionally set in a bit assembly having a bit holder that is retained within a bore of the bit holder block. Bits can include an insert having a conical cutting tip that is mounted in a recess in a frustoconical forward body portion of the bit. The insert can be made of a hardened material and/or can be surrounded by a hardened annular collar that provides added wear resistance to the cutting tool. The insert is further protected by a metal or steel sleeve. The cutting tool can include a solid generally cylindrical shank that extends axially rearwardly from the forward body portion. The bit fits in a central bore of the bit holder. As described, these bit holders are frictionally seated in the bores of their respective bit holder blocks mounted on the drums. These bit holders are not held in the bores of their respective bit holder blocks by retaining clips or threaded nuts, thereby providing easier removal and replacement once the bit holders are worn through use or broken due to the harsh road degrading environment.
Historically, these bits and bit holders have been made of steel with hardened metal or tungsten carbide tips or collars to lengthen the useful service life of the bit holder. Heavy duty road milling, mining, and trenching operations, however, impose much more wear and tear than the currently used industry standard bit holders can handle. The forces and vibrations exerted on the bit assemblies from the harsh road degrading environment may cause the bit holder to move within the bore of the bit holder block. Individual bits may wear or be broken off of their shanks because of the harsh environment and may also lead to the need to replace the bit holder.
Recently, materials harder than tungsten carbide, such as polycrystalline diamond or the like, have been used in certain road milling operations, notably the degradation of asphalt layers on long roadway stretches. While the hardness of the polycrystalline diamond tip lengthens the useful life of the combined bit and bit holder, the polycrystalline diamond tip of the combination is so brittle that it is generally not suitable for use in degrading concrete highways or curved highway stretches, such as cloverleafs and the like.
To prolong the life of the polycrystalline diamond tip bit assembly and prevent damage to the bit assemblies in heavy duty operations, a heavy duty combination bit and bit holder is provided that is sturdy enough to withstand the forces found when degrading or breaking up the surfaces of not only macadam (asphalt) roadways but also concrete roadways. Additionally, the metal sleeve supports both transverse and angular loads on the vertically exposed portion of the insert, which shields the forward end of the insert. The addition of this metal sleeve offers support and allows greater extension of the insert from the forward end. Most importantly, the metal sleeve significantly speeds the heat transfer in the induction brazing process and limits the polycrystalline diamond from excessive heat buildup which reduces degradation. The heat generated when the polycrystalline diamond is removing macadam, for example, dissipates faster through the metal sleeve, which will significantly increase the useful service life, by at least double, of a polycrystalline diamond insert brazed atop of a tungsten carbide bolster.